1. Field of the Invention:
This invention relates to a pneumatic module that allows a gradual adjustment of the level of air pressure pulses injected into the piston of any of the regular pneumatic vitrectomy probes available for intraocular microsurgical procedures.
2. Prior Art:
Until 1958 very few opthalmologists would risk surfically manipulating the vitreous gel. Although the vitreous gel comprises 70% of the eye weight and volume, it was considered a "prohibited area" for the great majority of eye surgeons because most interventions in which vitreous loss occurred were followed by complications such as corneal edema, glaucoma, retinal detachment, intraocular hemorrhage and even atrophy of the ocular globe.
The vitreous gel is composed of 99% water, and the remaining 1% includes two main components: collagen fibers and hyaluronic acid. The vitreous owes its characteristic consistency to its sincicial structure in which long collagen chains form the frame for dispersion of hyaluronic acid molecules. The last two components, collagen and hyaluronic acid, are bound in the vitreous gel to large amounts of water. This fact and the absence of vessels in the vitreous space guarantees the excellent light transmission properties and the inelasticity of the gel. The external surface of the vitreous is called the hyaloid membrane, a form of densified vitreous gel, which is in contact with the following eye structures: the posterior capsule of the crystalline, the pars plana epithelium, the retina and the optic nerve.
The many instances of light being prevented from normal focusing on the retina due to vitreous alterations have long stimulated the imagination of eye doctors on how to intervene upon these altered states. Examples of vitreous alteration are vitreous bands causing traction and detachment of the retina, vitreous opacification due to the inflammatory or infectious processes, presence of foreign bodies in the vitreous space, eye perforations causing vitreous loss, vitreous band formations and many other circumstances.
In 1958 Shafer tried to substitute diseased vitreous (D.M. Shafer, "The Treatment of Retinal Detachment by Vitreous Implant," 61 Transactions American Academy Ophthalmology Otolaryngol. pp. 194-200 (1958). An instrument for cutting vitreous bands was devised by Michaelson in 1960 (Michaelson, "Transcleral Division of Mid-Vitreous Membrane Under Visual Control," 44 British Journal of Ophthalmology pp. 634-635 (1960). The replacement of vitreous by silicon oil was performed in 1964 by Cibis, (P. A. Cibis, "Vitreous Transfer and Silicon Injection," 68 Transaction American Academy Ophthalmology Otolaryng., pp. 983-997 (1964). Freeman, Schepens and Anastopoulus started with their vitreous scissors in 1967 ("Vitreous Surgery--II., Instrumentation and Technique." 77 Arch. Ophthalmol. pp. 681-682 (1967).
The first successful and efficient mechanism for closed intraocular microsurgical procedures and vitreous cutting and removing was developed by Machemer, Buettner and Norton in 1970, with the vitreous cutter-sucker infusion instrument (Vitrectomy--American Academy of Ophthalmology and Otolaryng--Las Vegas 1970). Machemer was the first to conceive the idea of producing small pars plana incisions in order to introduce his probe for vitreous surgery. After the work of Machemer, other models were created, but basically all these probes together with their activating units and accessories are basically intended for cutting and removing abnormal vitreous or other unwanted tissue or substance (e.g. blood, fibrous tissue) from the vitreous cavity while keeping the intraocular pressure carefully controlled to minimize possible trauma for the globe. When the surgeon finds it necessary to remove the diseased vitreous or other extraneous matter from within the eye, this removal must be accomplished without damage to the retina, to the optic nerve or to their associated blood vessels. This is no easy task as the vitreous cannot be cut by a scalpel or other similar instrument because it is relatively tough and simply folds over the edge of the knife and refuses to be severed.
One can cut the vitreous gel in several ways:
(a) Mechanically, in which the apposition of two concentric needles, the inner one having a chopping, rotating or oscillating action against the outer tube cuts the vitreous gel and strands. After being cut the vitreous to be removed is drawn inside the inner tube by way of suction and in this way eliminated from the eye.
(b) Ultrasonically using phacoemulsification units, which emulsify and than aspirate the diseased vitreous (L. J. Girard, "Ultrasonic Fragmentation for Vitrectomy and Associated Surgical Procedures," 81 Trans. Am. Acad. Ophthalmol. Otol. p. 432 (1976).
(c) With laser energy (for example, cutting vitreous strands with the YAG neodymium laser).
In the previously mentioned Machemer instrument, the two tubes are mounted concentrically, both having a hole close to the end. The inner tube of the Machemer instrument rotates against the inner surface of the outer, stationary tube. At the moment the two holes appose one another, vitreous is sucked inside the internal needle. In this model the rotating or oscillating movement of the internal needle might cause problems due to a possible traction it can produce upon the collagen fibers, which can result in a pulling effect and injury to the retina. In the Machemer model a small electric motor at the rear end of the probe creates the rotating movement. In the vitrectomy probe described by Peyman ("Experimental Vitrectomy," 86 Arch Ophthal. p. 548 (Nov. 1971)), the inner tube oscillates and chops vitreous. A small, electric solenoid oscillates the inner tube. A few modifications and improvements in the probe models have been introduced with the passage of time, resulting in the appearance of new models:
(a) Chrome-plated internal needle, with a very fine tip finishing (the S.I.T.E. model, J. Federman, 7 Ophthalmic Surgery p. 82 (1976).
(b) Internal needle that has a precision fit as in the Grieshaber model, instead of being spring loaded as in the Machemer model, (Grieshaber-Schepens vitrectomy unit)
(c) Use of a rotating helical trunco-conical shaped internal needle precision fitted to the external needle, so that both tubes have a cutting blade surface, promoting a self-sharpening action and therefore reducing the wear of the cutting surface of the internal needle-Vitreous Nibbler (F. I. Tolentino, et al., "Vitreous Surgery--XII New Instrumentation for Vitrectomy," 93 Arch. Ophthalmol., p. 667 (1975).
A rotary cutting element is present in most of the previous vetrectomy instruments described. Rotary instruments, however, tended to have undesirable pulling or shearing effects on the tissue being severed. Efforts to avoid these effects led to the development of linearly reciprocating cutting instruments, an early sample of which is the initial model described by Peyman at al. 86 Arch. Ophthal. p. 548 (1971), in which cutting is performed by the chopping action of the sharpened end of the inner tube against the plane interior end of the outer tube. Suction applied to the inner tube causes the severed tissue to enter the hole in the outer tube, thus removing the severed bits of tissue from the eye. The necessary powered reciprocation of the inner tube relative to the fixed outer tube is provided by a small electrical solenoid, the oscillation rate of which can be varied. A description of this handpiece can also be found in Peyman, U.S. Pat. No. 3,776,238.
Although electrical solenoid devices provide a readily adjustable source of linear reciprocating motion, they also possess numerous drawbacks that limit their utility in a surgical environment. They tend to be relatively heavy, for example, which renders the handpiece somewhat incovenient to manipulate. During sustained operations, solenoid devices also tend to generate significant amounts of heat, which can damage delicate tissues. Moreover, since the solenoid is an integral part of the handpiece and must be supplied with an electrical current, an additional hazard is present.
Pneumatic power sources possess none of the foregoing disadvantages. Pneumatic devices are readily adaptable to linear reciprocating operations, do not inherently generate heat, and can be constructed from light weight materials. To the extent that electrical controls are necessary, they can be confined to a pneumatic power unit that is connected to the handpiece by an insulating pneumatic supply line and are well isolated from the surgical field. Pneumatic devices also tend to produce more evenly modulated power pulses than electrical solenoid devices. These factors make pneumatic devices ideal as power sources for powered vitrectomy instruments, and a number of pneumatically operated handpieces have been developed for ophthalmic use. With the continual refinement of endophthalmic surgical techniques and the proliferation of specialized reciprocating instruments based on the previously described prototype of Peyman, there has arisen a need for a pneumatic handpiece which is compact for convenient manipulation during delicate surgical procedures, simple in construction, safe to operate in a surgical environment, versatile in the sense of accommodating a number of different endophthalmic instruments in an interchangeable manner an reasonably priced.
One known type of pneumatic handpiece is described in O'Malley U.S. Pat. Nos. 3,815,604, 3,884,237 and 3,884,238. The disclosed handpiece consists generally of a cylindrical housing, an end cap for receiving one end of a projecting sharp-edged stainless steel tube, and a piston within the housing for providing reciprocating motion to a second stainless steel tube coaxially arranged within the first tube. The inner tube is sharpened at its distal end and the outer tube is provided with a lateral sharpened distal opening to form a push-type cutter. The inner tube is attached directly to the piston and extends axially through to the opposite side thereof for connection to a flexible evacuation line which passes out the back of the handpiece. The alternate air and suction pulses required for reciprocating the piston are supplied through a large-diameter tube also connect to the back of the handpiece.
O'Malley also introduced the idea of creating separate entrances for the infusion and illumination devices and for the cutting probe. In this way the bi-manual techniques for endophthalmic surgery were started and were promptly accepted because they made possible a better control of the eye position in surgical procedures. The other advantage was the less deleterious effect it had upon the eye creating three small 1 mm incisions in the pars plana instead of large 4-5 mm holes for entrance of large probes concentrically mounted, containing in one block the irrigation, illuminating and cutting aspects. The capability of moving the cutting needle by pneumatic means proved to be superior because of its dependability and simplicity. As a result, pneumatic systems have replaced solenoid systems to move the cutting needle.
At least three of the major handpieces available in the market are propelled by pneumatic means: the Ocutome probe of O'Malley, the "Vitrophage" probe of Peyman and the "Microvit" probe of the Storz Company (Wang, "Microsurgical Instrumentation for Vitrectomy: Part II," 9 (1) Journ. Clinical Engin. pp. 63-71 (1984)). The main difference in the propelling aspect of those probes is the level of air pressure necessary to move their internal needles, and the mechanism of bringing the cutting needle back to the rest position after being moved forward. In the Ocutome probe, a dual spring and suction mechanism returns the internal needle to the rest position. The return in the Vitrophage probe depends entirely on a spring action.